This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Acetylcholine (ACh) binding protein (AChBP) is the model/surrogate of the extra cellular domain for the nicotinic ACh receptor/ion channel complex (nAChR). As with the extra cellular domain of nAChR, the AChBP has five binding pockets for ACh or drug. The AChBP is assembled with five homologous subunits (homo-pentamer) and the ACh binding site exists on the interface region between subunits.Mammalian selective nAChR agonist epibatidine (EPI) and insect nAChR selective agonist imidacloprid (IMI, the representative neonicotinoid insecticide) share some common structural moieties, but they are distinctly different in their physicochemical properties. EPI has a protonated nitrogen atom (cation), but IMI has a non-protonatable and electronegative nitro (NO2) pharmacophore. These structural differences determine the selectivity between mammals and insects. As a proof, if IMI loses the nitro group (desnitro-IMI), it is now a mammalian selective compound.Therefore, amino acid residue(s) interacting with the cation of EPI and the electronegative nitro oxygen(s) of IMI must be quite different even if both compounds are buried in the identical hole, i.e., the binding orientations are different. To approach the molecular basis for the selective interaction, we designed and prepared two types of photoaffinity ligands. Therefore, the experimental goals of this study involving the LC/MS/MS analyses are to identify specific amino acid residue(s) labeled by these two types of photoaffinity ligands.